Hi, I've had no problems with vegas pro 19 for years and yesterday it randomly had this error saying there is no valid license, so I reinstalled from the website here and ever since then, every time I open it it prompts me to reactivate the program with my license key and email and does it successful each time.
You'd need to contact MAGIX Support bas they may need to reset your VP119 licence on your MAGIX account or something like that. There's a Contact Support link at the bottom of the following support webpage: https://www.vegascreativesoftware.info/us/support/vegas-pro--1/